Patent specification SE 319 592 (U.S. Pat. No. 3,371,952) describes a log handling device of the type intimated above. The auxiliary gripping arm is operated hydraulically from the hydraulic circuit of the main gripping arms via a control valve or delay valve in such a way that the auxiliary gripping arm is moved after the main gripping arms have closed. The valve is arranged on a first hydraulic line which leads to the hydraulic cylinders which drive the main gripping arms and is activated only when a predetermined pressure has been reached in this line. When the main gripping arms close, the pressure in the first line builds up and, when it reaches the predetermined value, the valve opens and admits fluid into a second line which leads to a hydraulic cylinder for moving the auxiliary gripping arm.
One disadvantage of the log-handling device according to SE 319 592 (U.S. Pat. No. 3,371,952) is that, if a tree trunk comes to lie between the main gripping arms in such a way that they are not able to fully close, the pressure in the first line builds up all the same, and the auxiliary gripping arm will be moved. This can lead to one or more logs being dropped during transport, which would entail a risk of harm or damage for both personnel and equipment located along the movement path of the gripping arm.